1. Field of the Invention
The present invention relates to an apparatus and method for sensing the absolute position of an object by monitoring the magnitude and direction of movement of the object in relation to a known location. More specifically, the apparatus and method uses an indexed-encoder, such as an indexed-encoder wheel, indexed-encoder bar, indexed-encoder film, or the like, having a plurality of alternating opaque and transparent spaces and a single index opaque space, and a sensor device, which is capable of distinguishing the single index opaque space from any of the other spaces and counting the spaces after detection of the single index opaque space to determine the direction and incremental positional movement of the indexed-encoder, and thus determine the absolute position of an object which the indexed-encoder is used to monitor.
2. Description of the Related Art
Many devices currently exist which are capable of detecting the relative position of a moving object, such as a rotating shaft or encoder. For example, as described in U.S. Pat. No. 4,704,523 to Uchida, a rotary encoder device is used to detect the relative rotational position of a rotating shaft of a numerically controlled industrial machine. A rotating plate is mounted to an end of the rotating shaft, and includes a series of light shielding portions and light transmission portions that are disposed continuously along its circumference.
A pair of optical fibers are arranged to transmit light toward the rotating plate, and a plurality of corresponding optical fibers are arranged on the other side of the rotating plate to receive the light. As the rotating plate rotates, the light transmission portions allow light to pass from the transmitting optical fibers to the receiving optical fibers, while the light shielding portions prevent transmission.
Because the light emitting fibers and corresponding light receiving fibers are offset from each other, the order in which the light receiving optical fibers receive light from their respective light emitting optical fibers indicates the direction of rotation of the plate. Also, each period of light detection/non-detection indicates that a light transmission portion and light shielding portion has passed between a light emitting fiber and corresponding light detecting fiber. Accordingly, the relationship between the order in which the light is received by the optical fibers and the number of detection/non-detection conditions can be processed to determine the direction in which the plate has rotated, as well as the angular distance of rotation from a period beginning when the apparatus began to count the detection/non-detection conditions.
However, it is noted that all of the light transmission portions and light shielding portions of the rotating plate are of the same size. Therefore, it is impossible to detect the absolute rotational position of the rotating plate, because no point on the rotating plate acts as a reference position. Rather, the Uchida apparatus is only capable of detecting the relative position of the rotating plate or, in other words, the rotational distance that the rotating plate has rotated from a time when the apparatus began to count the detection/non-detection conditions.
Other relative rotational position sensor apparatuses are described in U.S. Pat. No. 2,685,082 to Beman and U.S. Pat. No. 4,496,926 to Kramer. Each apparatus includes a disc that is mounted to a rotating shaft. The disc includes uniformly sized openings and projections or light shielding areas. However, neither disk includes a reference portion. Hence, like the Uchida apparatus, these apparatuses also are incapable of determining the absolute rotational position of their respective rotating discs.
Other types of rotational position sensor apparatuses are described in U.S. Pat. Nos. 4,911,449 and 5,058,893 to Dickinson et al. Each of the Dickinson apparatuses employ a coding ring having a special pattern of slots that indicates a home or synch position of the coding ring. Furthermore, each coding ring includes a repeating pattern of narrow and wide openings about its circumference.
As the coding ring rotates, the special pattern and repeating pattern passes through an optical sensing device. The pattern of on and off conditions of the detection signal output by the optical sensing device is analyzed to determine the position of the ring and also its direction of rotation. That is, if the long "on" conditions precede the short "on" conditions of the detection signal, this indicates that the ring is rotating in one direction (e.g., clockwise). However, if the short "on" conditions precede the long "on" conditions, this indicates that the ring is rotating in the opposite direction. Furthermore, the number of combined long and short "on" conditions can be counted to determine how many positions the ring has rotated after the home or synch position has been detected.
Unlike the apparatuses described in the Uchida, Beman and Kramer patents, the Dickinson apparatus does not use offset light detectors, but rather, relies on a more complicated pattern of openings (i.e., the narrow and wide openings) to determine direction of rotation of the ring. This more complicated pattern makes the ring more difficult to manufacture, and thus, more expensive than a ring having a uniform pattern of light transmitting portions and light shielding portions. Also, because the ring must include a narrow opening and a wider opening for each position, as opposed to one opening for each position as in the Uchida, Beman and Kramer apparatuses, it is more difficult to increase the resolution of the ring in the Dickinson apparatus, because more space along the circumference of the ring must be occupied for each position to accommodate the wide and narrow openings.
Accordingly, a continuing need exists for an inexpensive, easy to manufacture apparatus that uses an encoder device to detect the actual absolute position of an object by monitoring the direction and magnitude of movement of the encoder device.